This invention relates to in-line coating of a continuously moving substrate, such as a tube or conduit, of the type used for applications such as metal fencing or electrical conduit. More specifically, this invention relates to galvanizing and overcoating of such substrates.
The art of forming and coating tubes and conduits for fencing and electrical conduit is an old art. Many manufacturing operations exist which use techniques decades old. As an example, modern galvanizing procedures have been described as the outdated inheritance of original hot dip galvanizing in which cold articles were dipped in heated zinc pots. See U.S. Pat. No. 4,352,838 at column 1, lines 13-19.
While the art is old, significant advances have been made by industry leaders. These advances include the advance of PCT Publication No. WO 93/00453 published Jan. 7, 1993, the advance of U.S. Pat. No. 5,506,002, issued Apr. 19, 1996 and the advance of U.S. patent application Ser. No. 08/287,856 filed Aug. 9, 1994. As reflected in these patents and publication, galvanizing of continuous tubes and conduits has progressed to the point of rapid speeds of the tubes and conduits to be galvanized, on the order of six hundred feet per minute. Galvanizing has also progressed through the elimination of secondary or elevated zinc containers in favor of zinc pumped through cross-tees, spray nozzles and drip nozzles. Zinc application dwell times have been reduced to tenths of seconds, and contact zones to inches.
Industry leaders have also advanced the application of non-metal coatings, as well, as shown in U.S. Pat. No. 5,453,302, issued Sep. 26, 1995 As in this patent, protective coatings are applied by vacuum coating apparatus.
Applications of coatings through alternate coating technologies have also been disclosed. As shown in U.S. Pat. No. 3,559,280 issued Feb. 2, 1971, U.S. Pat. No. 3,616,983 issued Nov. 2, 1971, U.S. Pat. No. 4,344,381 issued Aug. 17, 1982 and U.S. Pat. No. 5,279,863, issued Jan. 18, 1994, electrostatic coating has been considered one possibility. As disclosed in U.S. Pat. No. 3,559,280, electrostatic spray coating is accomplished after water spray, sizing, straightening, and drying, and in the multiple steps and locations of a spraying or coating section, a separate following baking or hardening chamber, a separate following air blower and a separate following water spray. As disclosed in U.S. Pat. No. 3,616,983, electrostatic powder coating is accomplished as an alternative to other coating methods after earlier application of liquid coatings, and after heating applied by an external heater. As disclosed in U.S. Pat. No. 4,344,381, electrostatic spray coating is accomplished in an inert atmosphere by organic solvent-based, liquid coating materials.
U.S. Pat. Nos. 3,122,114; 3,226,817; 3,230,615; 3,256,592; 3,259,148; 3,559,280; 3,561,096; 4,344,381; 4,582,718; 4,749,125; 5,035,364; 5,086,973; 5,165,601; 5,279,863; and 5,364,661, and PCT Publication No. WO 93/00453 are incorporated by reference.